As part of the action used to cast this spell, you must make a melee attack with a weapon against one creature within the spell’s range, otherwise the spell fails. On a hit, the target suffers the attack’s normal effects, and green fire leaps from the target to a different creature of your choice that you can see within 5 feet of it. The second creature takes fire damage equal to your spellcasting modifier.

Since the caster fits the bolded criteria, does this mean he must target himself if no other creatures are available?

It's not entirely clear the caster fits the bolded criteria due to "your choice". The question then is: "Can the caster choose not to choose a creature?"
– MrLemonJun 25 '16 at 9:42

You can always use the old bag of rats trick. Carry around a bag of rats (or just assume there's one within 5 feet of the target). Then, if you must choose a target, select a rat.
– EthanJun 28 '16 at 21:48

I think it's always worth throwing in the caveat that Mearls himself always reminds us: he is not the authoritative word on RAW, but is an authoritative word on designer intent.
– nitsua60♦Jun 27 '16 at 14:43

3

Jeremy Crawford tweets are official rulings, so saying that no tweets are is absolutely false.
– Derek StuckiJun 27 '16 at 21:43

The quote from the linked article is stronger evidence than you give it credit for, and the comment, "you can always choose nobody for an effect worded that way" would make the answer cogent in a general sense.
– Tim GrantJun 28 '16 at 11:42

@timster I'm kinda looking for that ruling myself since the book would be a better source, but so far coming up short.
– Premier BromanovJun 29 '16 at 15:59

It's up to the DM, there is no official ruling

As noted in Sent_'s answer, Mike Mearls tells us that you needn't target yourself.

While Mike Mearls is a lead designer of DnD 5e -- as nitsua60 reminds us -- he is not the authority of rules and rulings. Mike Mearls concedes that there is no printed rule on his ruling. Thus, his ruling is simply that: A ruling. It carries as much weight as your own ruling as far as rules-as-written goes.

This is what leads me to believe that you needn't target yourself, because if you had to, why not just make yourself not an eligible target? just nip it in the bud.
– Premier BromanovJun 29 '16 at 20:29

I agree, but strict RAW this is how it seems to work.
– Jason_c_oJun 29 '16 at 21:05

If a spell targets a creature of your choice, you can choose yourself, unless the creature must be hostile or specifically a creature other than you. If you are in the area of effect of a spell you cast, you can target yourself.

It seems to me that Green-Flame Blade is by definition used against hostile creatures.

Further, a caster decides whether he is affected by his own spells when he is in the area of effect. So even if the flame was coming back at him, he would not take the damage if he chooses not to.

Re: your last paragraph: where do you get that a caster can choose not to be affected by their own spell? RPGSE doesn't agree, for instance, and there's nothing in targeting rules to sustain that.
– nitsua60♦Jun 27 '16 at 14:47

@nitsua60 "If you are in the area of effect of a spell you cast, you can target yourself.", meaning that by default you probably don't include yourself if the effects are damaging?
– Alexis WilkeJun 27 '16 at 17:26

That sounds just like the argument Jonathon Wisnoski's making over on the question I linked--perhaps you should weigh in over there?
– nitsua60♦Jun 27 '16 at 17:30

Yes. That looks about right. The sculpt feature of the wizard would be the exact same thing as what I say here. Only that feature is limited to the caster by default and in case of a wizard with the evocation specialty, he can further protect other people.
– Alexis WilkeJun 28 '16 at 22:18

@nitsua60 Also, as per sent_'s answer, Mike Mearls clearly says that if the caster is given the choice, the caster can choose either way and therefore in this case can choose to not target oneself.
– Alexis WilkeJun 28 '16 at 22:33